1. Technical Field
This invention relates generally to detecting systems, and more particularly to a passive millimeter wave detector particularly well adapted for detecting the presence of moving or stationary vehicles for the purpose of controlling and monitoring traffic generally, as it moves over highways, airport runways, railroad-right-of-ways and sea lanes, or generally along any defined path taken by a vehicle under normal conditions of visibility as well as conditions of poor visibility.
2. Discussion
The ability to monitor traffic is becoming increasingly important as highways, expressways and airport runways become increasingly more heavily traveled and congested. While the following discussion will refer to highway conditions, it will be understood the invention can be useful anywhere vehicles move along fixed and predetermined routes.
Most prior known systems use magnetic induction loops buried just beneath the roadway surfaces and respond to passing vehicles by sensing a change in the resonant frequency in the circuit containing the loop. Such systems are expensive and inconvenient to install and maintain. Furthermore, such systems may be impossible to implement in certain applications, such as in connection with monitoring sea traffic lanes. Accordingly, there is a need for new detection and monitoring systems that are easy and inexpensive to install, maintain and operate without disturbing the roadway and delaying traffic in the process.
One form of system currently being produced which does not require a sensor buried in the roadway uses a Doppler radar that directly measures the vehicle velocity as a frequency shift using the outgoing RF radiation and the reflected signal. These systems suffer from the deficiency of being unable to detect stopped vehicles which produce no frequency shift in the reflected signal. These systems typically rely on irradiating an area with RF radiation which may interfere with other radar or communications equipment.
Ultrasonic waves and reflected light have been used as sensors in place of RF radiation but are known to have reduced operating capability in bad weather.
Wide area detection techniques using video cameras have been established. While these systems can monitor multiple lanes of traffic over extended distances using a single camera, the images must be computer-processed to extract the desired data. Image processing is complex and subject to errors due to the cluttered scenes (e.g., car headlights at night versus sunlight during the day, highlights due to direct reflection of sunlight from shiny surfaces, shadows due to passing clouds, etc.). Furthermore these systems perform poorly under conditions where there is low visibility due to fog or blowing dust.
While the above approaches have met with some success, each nevertheless is subject to the drawbacks of high lifecycle costs, inability to sense stationary vehicles, inability to function in bad weather conditions, emission of undesirable RF radiation, or the introduction of significant errors due to complex data interpretation.
Applicants have invented a sensing device for the passive detection of the naturally occurring millimeter wave ("MMW") radiation which forms part of the electromagnetic spectrum, from about 30 to 300 GHz (wavelengths of 1 to 10 millimeters). This approach overcomes all of the aforementioned limitations to provide comprehensive traffic monitoring data including vehicle count, stopped vehicles, and the vehicle's speed and length.
The phenomenon of passive millimeter wave detection is a function of the radiation wavelength. At micro and millimeter wavelengths that correspond to windows of high atmospheric transparency (e.g., 10 GHz, 35 GHz, 95 GHz, and 140 GHz), the sky appears very cold (tens of degree Kelvin) regardless of whether it is day or night. This long wavelength radiation is only slightly attenuated by atmospheric fog or cloud droplets and hence the apparent sky temperature is insensitive to weather conditions. Objects that are observed at MMW wavelengths are characterized as having a "radiometric temperature" which is the equivalent of the black body temperature required to produce an equivalent radiated power. Metallic objects are highly reflective and virtually non-emissive at these wavelengths and therefore radiate very low levels of radiation, independent of weather or time of day. Such objects assume the radiometric temperature of whatever they reflect. Rough nonmetallic solids, on the other hand, are highly emissive and emit higher radiated power, and are characterized by a radiometric temperature nearly equal to its physical temperature. Water is intermediate in reflectivity and its radiometric temperature represents a mix of reflected and emitted radiation.
With particular regard to a traffic sensing system, it will now be appreciated that the difference in effective radiometric temperatures and radiated power per unit area between a vehicle and the surface upon which it is traveling, such as asphalt, forms the basis for the passive millimeter wave traffic sensor. For example, asphalt will radiate a higher power per unit area and will appear to be at a higher radiometric temperature. A metallic vehicle, however, radiates a lower power per unit area and it will appear to be at a lower radiometric temperature. A sensor which can measure the amount of radiated power coming from a specific area on the road surface will detect the passage of the vehicle as a change in the amount of power received. The effect is insensitive to daylight, night, clouds, fog or precipitation. If the vehicle is stopped, the decrease in the received signal persists as long as the vehicle is present, so that detection of the vehicle presence does not depend on vehicle motion. By sensing the rate of change of the received power as the vehicle enters and leaves the field of view of the sensor, the speed of the vehicle can be estimated. By measuring the duration of the period during which the vehicle signature is present, and from the previously known vehicle speed, the length of the vehicle can also be estimated.
It is therefore a principal object of the present invention to provide a passive millimeter wave sensor capable of detecting objects such as land, sea and air vehicles within a predetermined field of view.
It is another object of the present invention to provide a passive millimeter wave sensing device to detect the presence of millimeter wave signals emanating from a predetermined field of view, such as a lane of a highway, airport runway or sea lane.
It is yet another object of the present invention to provide a passive millimeter wave sensing device which can detect millimeter wave signals within a predetermined field of view and can operate under a wide range of weather conditions in a wide range of geographical locations and seasons of the year.
It is still another object of the present invention to provide a passive millimeter wave sensing device for detecting millimeter wave signals within a predetermined field of view for the millimeter wave radiation which may be mounted above ground at locations proximate to where the subjects are to be monitored, and which can provide information relating to the speed and length of moving objects, as well as the presence or absence of objects, whether stationary or moving, as well as a total count of the objects (i.e., vehicles) which pass through the field of view of the sensing device.